Gravity attains a temperature of 700°c. The melting temperature

Gravity die casting is a manufacturing
process for producing accurately dimensioned, sharply defined, smooth or
textured-surface metal parts. It is accomplished by gently pouring molten metal
into reusable metal dies under the force of gravity. The term “die
casting” is also used to describe the finished part(“Aluminium
Castings | Gravity Die Castings | Arrow Butler Castings,” 2018). The piston alloys were prepared by
gravity die casting technique using a diesel fired tilting furnace. The as
received materials such as commercial Al-6061 alloy, Al-50% Si, Al-50% Cu and
Al-20% Ni master alloys were used for the preparation of the alloys. Al10%Sr
master alloy was used for modification of piston alloys. The preheated ingots
were charged in to the furnace when the crucible attains a temperature of
700°c. The melting temperature was maintained at 750±5°C. The molten melt was
continuously degassed by bubbling Ar gas into the melt. The molten metal was
poured at temperature of 720-730°C in to the open preheated metal mold and
after solidification, mold allows to water quenching.

1.1.1.          
Heat Treatment Process

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The T6 heat treatment process were
carried out by using electrical muffle furnace. Solutionizing of the specimens
were done at a temperature of 450±5°C for six hours, followed by water
quenching at a temperature of 20°C. The ageing or precipitation treatment is
carried out at a temperature of 180°C for 10hrs and the specimens were removed
from the furnace and followed by air cooling (Zeren,
2007).

1.2.      
MICROSTRUCTURAL STUDIES

 

The structural features of the alloy have
been characterized usingOptical emission spectroscopy(OES), Optical microscopy
(OM), Scanning Electron Microscopy (SEM) and EnergydispersiveX-ray spectroscopy
(EDS).

1.2.1.
Optical emission spectroscopy

 

Optical emission spectroscopy using arc
and spark excitation (Arc Spark OES) is the preferred method for trace metal
analysis to determine the chemical composition of metallic samples. This
process is widely used in the metal making industries, including primary
producers, foundries, die casters and manufacturing. Due to its rapid analysis
time and inherent accuracy, arc spark optical emission spectroscopy systems are
most effective in controlling the processing of alloy(Smelting,
1998).The chemical compositions of the piston
alloys were measured by SPECTROMAXx Optical Emission Spectrometer using arc
spark excitations.

1.2.2.
Optical Microscopy

 

The
metallographic specimens are cut from the castings using band saw with
carborandum wheels. The specimens are polished using silicon carbide emery
paper of sizes varying form 80, 200, 400, 600, 800 and 1200 grit sizes. The
specimens are washed thoroughly using liquid soap and water while going to next
paper size and the orientation of polishing surfaces was changed by 90°. After
completion of the paper polishing the specimens was polished by disc polisher
using 400 mesh Al2O3 powder with water.Then the specimens
were polished using “Silvo” and final polishing was done using 0.25?m
diamond paste in a rotating wheel (around 500 rpm) with a gentle applied
pressure. The specimens were washed very well with liquid soap solution. The
specimens have been observed and analysed in etched (using Keller’s reagent
(2.5% HNO3+ 1.5% HCl + 1%HF + 95%H2O) solution) condition. The microstructural
features of alloy specimens were observed using Olympus CX21i Microscopeat different locations with varying
magnifications.

1.2.3.
Scanning Electron Microscopy (SEM)

 

The scanning electron microscope (SEM)
uses a focused beam of high-energy electrons to generate a variety of signals
at the surface of solid specimens. The signals that derive from electron-sample
interactions reveal
information about the sample including external morphology (texture), chemical
composition, and crystalline structure and orientation of materials making up
the sample. In most applications, data are collected over a selected area of
the surface of the sample, and a 2D image is generated that displays spatial
variations in these properties(Swapp,
2006).Scanning electron microscopy was
performed in heat treatedpiston alloy specimens. The piston alloys sampleswere deep etched using hydrofluoric acid. A ‘JEOL JSM 6460’ scanning electron microscopewasused for the analysis.